Journal of Membrane Science 173 (2000) 35–52
Experimental configuration and adsorption effects on the permeation of C4 isomers through ZSM-5 zeolite membranes Christopher J. Gump, Xiao Lin, John L. Falconer∗ , Richard D. Noble Department of Chemical Engineering, University of Colorado at Boulder, Boulder, CO 80309-0424, USA Received 7 October 1999; received in revised form 2 February 2000; accepted 7 February 2000
Abstract Butane isomer permeation through two types of ZSM-5 zeolite membranes was studied as a function of temperature using three experimental configurations: pressure drop, sweep gas on the permeate side, and vacuum on the permeate side. For one type of membrane, which has a significant permeation through small non-zeolite pores, separation occurs by preferential adsorption and pore blocking. For these membranes, n-butane/i-butane separation selectivities are higher than ideal selectivities, and are much higher when a pressure drop is used (maximum selectivity of 140). For single gases, the larger i-butane molecule permeates faster than n-butane in this type of membrane with the pressure drop method, apparently because the i-butane coverage gradient is larger than the n-butane gradient. When a sweep gas is used, n-butane permeates faster. For the other type of membrane, which has permeation mostly through zeolite pores, separation is controlled by differences in diffusion rates and adsorbed coverages. The single-gas and mixture permeances for these membranes are similar for each gas for the pressure drop and sweep gas methods. Ideal selectivities increase in the order pressure drop
1. Introduction Zeolites, which have pores that are the size of small molecules and adsorb some molecules strongly, are used in adsorption-based separation processes [1,2]. Zeolite membranes, which consist of a thin layer of zeolite crystals, have the potential to separate mixtures of compounds that are otherwise difficult to separate, ∗ Corresponding author. Tel.: +1-303-492-8005; fax: +1-303-492-4341. E-mail address:
[email protected] (J.L. Falconer)
including organic isomers. In addition, their inorganic crystalline structure gives them mechanical strength as well as thermal and chemical stability. Much of the existing experimental work on zeolite membranes used MFI crystals, which include ZSM-5 and its pure silica analog, silicalite-1. The MFI pore structure consists of straight, circular pores (0.54 nm×0.56 nm) running perpendicular to sinusoidal, elliptical pores (0.51 nm×0.54 nm), as measured by XRD [3]. These pores are similar in size to many organic molecules, and have been shown to separate mixtures of organic isomers [4–7]. In addition to the zeolite pores, zeolite
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 5 4 - 9
36
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
membranes may contain non-zeolite pores that result from intercrystalline grain boundaries or regions of amorphous silica. These pores can differ in size and adsorption properties from zeolite pores, but are small enough in some membranes to allow separations by preferential adsorption and pore blocking at lower temperatures [8,9], or by molecular sieving [10]. Since only a thin layer of intergrown zeolite is required to form an effective membrane, the crystals are usually deposited on a mesoporous support, which provides mechanical strength. Support materials include ␣- or ␥-alumina, stainless steel, and clay. 1.1. Permeance measurement methods Comparisons for zeolite membranes of permeances and separations that have been reported in the literature must be done carefully since different experimental systems and methods are used by different investigators. Two common membrane geometries have been studied: disks [11–13] and tubes [4,14–16]. Because of the larger surface area for a given volume, tubular membranes are more desirable for industrial scale-up. The zeolite layer on tubular membranes can either be synthesized on the inside [16] or the outside [17] of the support. Gas permeances are typically measured in either pressure drop or Wicke–Kallenbach (WK) modules. Pressure drop modules impose a total pressure gradient across the membrane, and utilize the pressure drop as the driving force for permeation. The pressure gradient can either be generated by pressurizing the feed [18] or by evacuating the permeate side of the membrane [13]. For single-component permeation, the module may be dead-ended, but for mixture separations, the feed must flow continuously [8]. WK modules impose a concentration gradient across the membrane by flowing a sweep gas on the permeate side to remove the permeating species [12,19]. Both sides of the membrane are held at the same pressure, eliminating viscous flow. The feed gas flows continuously to replace any permeating species and to remove any back-diffusing sweep gas. Both pure component and mixture permeation can be studied with this system. Van de Graaf et al. [20] found that the orientation of the membrane greatly affected the separation behavior. Concentration polarization effects resulted in a complete loss of selectivity for mixtures of methane/ethane when the zeolite layer was facing the permeate side.
1.2. Effect of configuration The measurement method affects the values of the permeances and the selectivities because changing the pressures on the feed and the permeate side of the membrane changes the coverages or occupancies in the membrane pores and thus changes the concentration gradient driving force. Similarly, the separation selectivity depends on the measurement method because of competitive adsorption, which is a function of pressure. Since zeolite membranes are prepared by different methods in different laboratories, the differences in their permeation properties can be due to either the intrinsic membrane properties or the measurement methods. Using a WK module, Lovallo et al. [21] and van den Broeke et al. [22] found that CO2 permeated through silicalite-1 disk membranes faster than CH4 as a pure gas between 380 and 480 K. For other silicalite-1 disk [23] and tube [24] membranes, CH4 permeated faster than CO2 when a pressure-gradient experimental configuration was used. Similar behavior has been seen for single-gas permeation of n-butane and i-butane in silicalite-1 membranes. Using a WK module, Vroon et al. [5] and Geus et al. [25] measured n-butane single-gas fluxes that were 10–100 times larger than i-butane. Coronas et al. [16], using a pressure gradient and different membranes, observed that i-butane permeated faster than n-butane as a single gas above 340 K. These differences could be due to the measurement method, a fundamental difference in the silicalite-1 membranes prepared in different laboratories, or to a combination of these causes. Van de Graaf et al. [19] investigated the effects of these experimental configurations by using single-gas permeation of short-chain linear alkanes. They used a disk membrane in three experimental designs: a WK module with both sides at atmospheric pressure, and two batch methods in which the feed was pressurized and the permeate was either evacuated or was at atmospheric pressure. They concluded that, at low temperatures (high surface coverage), transport across the membrane was controlled by the adsorbed phase concentration and not by the partial pressure difference. Propane permeated fastest in the batch method when the permeate was evacuated. At low feed pressures, propane permeance was higher when using the WK module than in the batch method with atmospheric pressure on the permeate side, but as the
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
pressure increased, both methods yielded the same permeances. As the temperature increased, the increase in diffusion rate was offset by the decrease in surface coverage, resulting in a maximum in propane permeance with temperature. At high temperature, both batch methods produced similar permeances with the evacuated permeate configuration having slightly but consistently higher values. The WK permeances were lower than those measured with the batch method, due to the back-diffusion of the sweep gas, and the resulting additional resistance to transport. 1.3. Permeation models Most models of transport through zeolite membranes incorporate a surface diffusion term based on a Maxwell–Stefan formulation [26–28]. In such a model, the flux of a species through the membrane depends on the concentration-independent Maxwell– Stefan micropore diffusivity of the species, the thermodynamic factors of the permeating species, and the gradients of the surface coverages inside the membrane pores [29]. These terms are represented in Eq. (1): N = −ρε –D 0
dq dx
(1)
where N is the flux through the membrane, ρ is the density of the zeolite, ε is the porosity of the mesoporous support, –D is the Maxwell–Stefan diffusivity, 0 is the thermodynamic factor, q is the adsorbed phase concentration in the membrane, and x is the distance through the membrane from the feed side. The thermodynamic factor, 0, can be calculated using Eq. (2) and the appropriate adsorption isotherm relating pressure, P, to coverage, q: 0=
q ∂P ∂ lnP = ∂ lnq p ∂q
(2)
Using a model derived from the Maxwell–Stefan equations as applied to surface diffusion, Krishna and van den Broeke [29] modeled the transient flux of mixtures of methane and n-butane, and hydrogen and n-butane across a silicalite-1 membrane. In both cases, the smaller, weaker adsorbing component (methane or hydrogen) exhibited a maximum in flux at early times. At longer times, the n-butane flux increased to
37
a higher steady state value than the other component. These modeling results match the experimental data from Bakker et al. [30]. Van de Graaf et al. [19,20] modeled the separation of n-butane and i-butane as a function of permeate pressure, and found that flux and permselectivity decreased as the permeate pressure approached the feed pressure. They also modeled the effect of sweep gas flow rate on the flux and the selectivity of a mixture of methane and ethane. Increased sweep gas flow rates affected the flux of ethane more than the flux of methane. Therefore, the selectivity increased with increasing sweep gas flow rate. In the current work, three membranes synthesized by two procedures were used to explore the effects of experimental configuration and membrane properties. Since the effect of measurement method may be different for membranes with different properties and pore structures, membranes with significantly different permeation behavior were used. Two membranes were produced by a procedure similar to that described by Coronas et al. [16]. This type of membrane has been reported to separate organic isomers, and the separation selectivities for n-butane/i-butane are high. The N2 /SF6 ideal selectivities are also high (100–400). In contrast, the ideal selectivity for n-hexane/2,2-dimethylbutane is low (2–3). These membranes were concluded to contain nanosized non-zeolite pores capable of separating mixtures by preferential adsorption and pore blocking. The third membrane was synthesized by procedures described by Tuan et al. [31]. These membranes have lower N2 /SF6 ideal selectivities, but have high n-hexane/2,2-dimethylbutane ideal selectivities and separate C4 and C6 isomers effectively at significantly higher temperatures than the two membranes prepared with the procedure of Coronas et al. [16]. Single-gas permeation and mixture permeation of n-butane and i-butane through these membranes were measured as a function of temperature using three experimental configurations. The driving force for permeation was supplied either by a 138 kPa pressure gradient with the permeate at either atmospheric pressure or vacuum, or by a concentration gradient created through the use of a helium sweep gas in a configuration similar to a WK module. The measurement method has a significant effect on the ideal and mixture selectivities for both types of membranes. The experimental results were fitted to a Maxwell–Stefan
38
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
transport model in an attempt to explain the effect of measurement technique on the permeation results. 2. Experimental methods 2.1. Membrane preparation Membranes were prepared by in situ crystallization onto mesoporous supports using two methods. The first synthesis technique, used to make membranes C1 and C2, was based on a modified silicalite-1 gel of Grose and Flanigen [32], with an Si/Al ratio of 100 and a molar composition of 1 TPAOH:21 SiO2 :987 H2 O:3 NaOH:0.105 Na2 Al2 O4 ·3 H2 O, where the Si source was Degussa Aerosil 300 and TPAOH is tetrapropylammonium hydroxide. One membrane was grown on an ␣-alumina tube (US Filter, 4.7 cm long, 0.7 cm ID, 200 nm pores), and another was grown on an ␣-alumina tube with a 5 m thick inner layer of ␥-alumina (5 nm pores). Electron probe microanalysis (EPMA) performed by Coronas et al. [16] on membranes grown by similar procedures on alumina supports showed that the high alkalinity of the synthesis gel dissolved a portion of the support, resulting in a membrane with an Si/Al ratio closer to 30. Approximately 1 cm on each end of the support was glazed (GL 611A, Duncan) to prevent membrane bypass. The high temperature treatment required to set the glaze (1070 K) altered the ␥-alumina layer. Supports in which the ␥-alumina layer was deposited before the glazing procedure had a nitrogen flux 140 times larger than those in which the ␥-alumina layer was deposited after the glazing procedure. The supports were boiled in water for 1 h and then dried at 443 K before membrane preparation to remove any particulates or other contaminants. The zeolite layers for membranes C1 and C2 were synthesized using the Procedure 1 method described by Coronas et al. [16] as modified by Lin et al. [33]. One end of the support was sealed with Teflon tape and a Teflon cap. The inside of the support tube was filled with approximately 2 ml of gel, and then the top end was taped and capped. The capped tube was then placed inside a Teflon-lined acid digestion bomb (Parr) along with approximately 1 ml of water. The bomb was sealed and placed in an oven at 443 K for 15 h to synthesize the first layer. The membranes were dried at 443 K, and were found to be permeable to N2 at
room temperature with a 138 kPa pressure gradient. A second layer was synthesized on membrane C1 for 8 h, after which it was impermeable to N2 . Membrane C2 was boiled in water for 1 h prior to the synthesis of the second layer to remove any loose zeolite crystals and other contaminants. It, too, was impermeable to N2 after the second layer was synthesized. As shown by SEM photographs and EPMA, this synthesis technique produces membranes consisting of a continuous layer of zeolite crystals on the inside edge of the mesoporous support [16]. Membrane T1 was synthesized by a different procedure, using a less viscous, alkali-free gel with a higher Si/Al ratio (600). The support was stainless steel with 500 nm pores (4.7 cm long, 0.95 cm OD, Mott Metallurgical) with short sections of non-porous stainless steel tubing welded onto each end for the o-rings to seal against. The molar composition of the synthesis gel was 438H2 O:19.5SiO2 :0.0162Al2 O3 :1TPAOH. The silicon source was Ludox-40 (40% SiO2 , DuPont), and the aluminum source was aluminum isopropoxide (98+%, Aldrich). A detailed description of the synthesis along with SEM photographs and EPMA cross-sections of typical membranes are given by Tuan et al. [31]. To prepare the membrane, one end of the stainless steel support was sealed with Teflon tape and a Teflon cap. The inside of the support was filled with synthesis gel and kept overnight at room temperature to allow the gel to soak into the support. The tube was refilled, taped and capped, and sealed in a bomb and placed in a 460 K oven for 22 h to form a precrystallization layer. A layer was then crystallized onto this layer at 460 K for 48 h. The support was cleaned and dried overnight. After being dried at 373 K, the membrane was impermeable to N2 . After synthesis, all membranes were calcined at 753 K for 8 h to remove the TPAOH template molecules from the pores. The oven was heated and cooled at 0.6 and 1.2 K/min, respectively, to minimize thermal stresses. Once calcined, the membranes were sealed in a dead-end permeation system to test their quality. The ambient-temperature, single-gas permeation rates of N2 and SF6 were measured at a feed pressure of 222 kPa and a permeate pressure of 84 kPa. Since N2 has a kinetic diameter smaller than zeolite pores and the diameter of SF6 is similar to the zeolite, the ratio of the two permeances was used as an indication of membrane quality.
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
2.2. Experimental configurations Three experimental configurations were used in the permeation experiments. The first was an imposed pressure gradient across the membrane. For these experiments, the feed and the permeate were maintained at 222 and 84 kPa, respectively. A second configuration also used a pressure gradient. The feed side was maintained at 138 kPa, while a vacuum pump continuously removed permeating species. The absolute pressure of the permeate varied somewhat with permeation rate, but was consistently lower than 0.5 kPa. The third configuration was similar to the WK arrangement [14,34]. Both sides of the membrane were maintained at 138 kPa. The feed consisted of either the pure gas or an equimolar mixture of the two components (no helium diluent). A helium sweep gas flowing at approximately 80 ml/min (STP) continually removed permeating species from the outside of the membrane. 2.3. Separation apparatus Experiments were conducted in a continuous-flow, steady-state permeation module described in detail elsewhere [35]. Two mass flow controllers regulated the pure component flow rates to generate the mixture feeds. The combined flow rate of the mixture feed was typically 40 ml/min (STP). The flow rates for the single-gas experiments were typically 20 ml/min (STP). The feed flowed axially through the membrane, with the permeate diffusing radially outward. Silicone o-rings sealed the membrane inside the module. The module was wrapped in heating tape and insulation, and a temperature controller maintained the desired temperature based on a thermocouple placed at the axial outlet of the membrane. Back-pressure regulators on both retentate and permeate streams allowed for the pressure of each stream to be set independently. For the experiments in which the permeate side was evacuated, a dual stage rotary vane pump was connected to the permeate side. The pressure of the permeate was measured using a vacuum pressure gauge. Since the permeate compositions could not be measured, the permeate flow rate and composition were calculated by mass balance. In order to improve accuracy in the mass balance, the total feed rate for the vacuum experiments was typically 20 ml/min (STP). This was about twice the highest permeation rate through the
39
membrane. When a sweep gas was used, the flow of helium was set to 80 ml/min (STP). The permeate and retentate streams were analyzed by an HP 5890 gas chromatograph. A 10-port pneumatic sampling valve was used to sample the streams on-line. The chromatographic separation was accomplished using a 6 ft Alltech GraphPac-GC packed column. Each permeance was calculated from an average of four samples taken from the retentate and permeate streams. The calculated concentrations from the four samples at a given set of conditions typically varied by less than 2%. The volumetric flow rates of the retentate and permeate streams were measured at approximately 300 K and 84 kPa using soap film flow meters. 2.4. Procedure The membranes were calcined for 4 h at 673 K to remove contaminants prior to each experimental run using a given flow configuration (pressure drop, vacuum, or sweep gas). Each experiment started at either room temperature or 473 K, and the temperature was then stepped to the other end of the experimental range. Hysterisis effects were checked by returning to the initial temperature, but no hysterisis was seen, irrespective of whether the experiment was started at low or high temperature. Permeances were calculated as the flux of each component through the membrane divided by the driving force for permeation. Since the module has a cross-flow design, a log-mean pressure drop was used to calculate the driving force from the partial pressures. For both pressure drop and vacuum experiments, the permeate was considered to be well mixed. In the sweep gas experiments, the driving force was calculated between the partial pressures of the feed and the permeate, and the partial pressures of the retentate and the sweep (which was pure helium). Ideal and mixture selectivities were calculated as the ratios of the single-gas and mixture permeances, respectively.
3. Results Table 1 lists the support type, N2 permeance, and N2 /SF6 ideal selectivity for each membrane. The N2 and SF6 permeances were measured at 298 K with a feed pressure of 222 kPa and a permeate pressure of 84 kPa. Although the N2 /SF6 ratio depends strongly
40
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
Table 1 Membrane single-gas permeation properties at 298 K using a 138 kPa pressure drop (permeate at 84 kPa) Membrane C1 C2 T1
Support type
␣-Alumina ␥-Alumina Stainless steel
N2 permeance×107 mol/(m2 s Pa)
Ideal selectivity (N2 /SF6 )
8.8 4.3 0.38
240 310 19
on the synthesis procedure and experimental configuration (sweep gas or pressure gradient) [8,33], it is a useful indication of membrane properties. For good quality membranes prepared by the method of Coronas et al., the N2 /SF6 ideal selectivities are typically 200–300 [8,9]. For the alkali-free membranes prepared by the method of Tuan et al., the ideal selectivities are much lower (∼20) for an effective membrane [31]. Similar low N2 /SF6 ideal selectivities have been reported for effective silicalite-1 or ZSM-5 membranes prepared by others [36,37].
mers are about the same, and the selectivity is just slightly greater than 1. The single- and mixture-gas permeances for membrane C1 using the sweep gas configuration (Fig. 1b) exhibit quite different behaviors from those seen with the pressure drop configuration. The single-gas permeance of n-butane is higher than the permeance
3.1. Membrane C1 Fig. 1a shows the single- and mixture-gas permeances on a log scale for the butane isomers through membrane C1 using the pressure drop method. At 300 K, n-butane permeates about 15 times faster than i-butane as a single gas. The n-butane permeance increases exponentially with temperature, but the i-butane permeance increases more rapidly at low temperature, so that, as a single gas, i-butane permeates as much as four times faster than n-butane above 340 K. In contrast to the single-gas behavior, membrane C1 is selective for n-butane in the mixture over the whole temperature range studied. For n-butane, the single-gas and the mixture permeances are almost identical. The i-butane permeance, however, is significantly different for the single- and mixture-gas measurements. In the mixture, i-butane permeance decreases as the temperature increases from 300 to 340 K, so that the membrane is selective for n-butane in the mixture even though i-butane permeated faster as a single gas. Between 373 and 403 K, n-butane decreases the i-butane permeance by almost three orders of magnitude. The i-butane permeance in the mixture only increases significantly above 400 K; it increases more than two orders of magnitude from 400 to 460 K. By 480 K, the permeances of both iso-
Fig. 1. Single-gas and mixture permeances for a n-butane/i-butane 50/50 mixture for membrane C1 using two experimental configurations: (a) a 138 kPa pressure drop (feed at 222 kPa) and (b) a sweep gas (both sides at 138 kPa).
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
of i-butane over the entire temperature range. Also, the n-butane single-gas permeance is higher than its mixture permeance over the entire temperature range, though it shows essentially the same temperature dependence. Neither of the n-butane permeances are simple exponential functions of temperature as they were in the pressure drop configuration. The n-butane single-gas permeances are higher when measured by the sweep gas method. The single-gas i-butane permeance for membrane C1 is almost an order of magnitude lower when measured by the sweep gas method, and it changes much less with temperature. In the mixture, the i-butane permeance is lower than its single-gas permeance by as much as one and a half orders of magnitude. The i-butane mixture permeance does not exhibit the low temperature minimum seen with the pressure drop method. Instead, the permeance increases over the entire temperature range, but the increase is less than two orders of magnitude. By about 490 K, the permeances of n- and i-butane are similar and the selectivity falls to about 1.5. The ideal and separation selectivities for membrane C1 calculated from the permeances in Fig. 1a and b are shown on a log scale in Fig. 2. The four n-butane/i-butane selectivities are around 10 at room temperature, but they differ dramatically at the other temperatures. The mixture selectivities measured by both methods increase with temperature and
Fig. 2. Ideal and mixture selectivities as measured using both a 138 kPa pressure drop (feed at 222 kPa) and sweep gas (both sides at 138 kPa) configuration for membrane C1 for permeances in Fig. 1a and b.
41
go through maxima, whereas the ideal selectivities decrease and go through minima. Note that the mixture selectivities obtained using a pressure drop are two to four times larger than those obtained with a sweep gas. The maximum separation selectivity is 100 for pressure drop measurements and only 23 with a sweep gas. In contrast, the ideal selectivities are as much as an order of magnitude lower when measured by the pressure drop method than when a sweep gas was used. Since the ideal selectivities are less than 1 above 340 K for the pressure drop experiment, the ideal selectivities are clearly not an indication of the separation performance for these membranes. For the pressure drop method, over the entire temperature range, the mixture selectivity is greater than 1, whereas for most of the temperature range, the ideal selectivity is significantly less than 1. In contrast, for the measurements made with sweep gas, the permeances of both isomers exhibit much less temperature dependence. The ideal selectivity with a sweep gas also decreases with temperature, but the membrane remains selective for n-butane. At high temperature, the ideal and mixture selectivities measured with sweep gas are almost the same, and are the same as the mixture selectivity measured by pressure drop. 3.2. Membrane C2 Membrane C2, prepared on a ␥-alumina support (glazed at high temperature) with intermediate boiling treatments in water between synthesis steps, exhibited a mixture permeance behavior similar to that of membrane C1, but its separation selectivities were higher. Fig. 3 shows the permeances of both isomers in a mixture using the pressure drop and sweep gas configurations. The n-butane permeance using the pressure drop method increases approximately exponentially with temperature. The n-butane permeance values are similar in magnitude to those measured for membrane C1, but the i-butane permeances are lower, and thus, this membrane exhibits higher separation selectivities. As for membrane C1, the i-butane permeances measured by the sweep gas method are significantly higher than those measured with a pressure drop over most of the temperature range. For the pressure drop method, the i-butane increases by almost three orders of magnitude over the temperature range.
42
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
Fig. 3. Mixture-gas permeances for a 50/50 n-butane/i-butane mixture using both a 138 kPa pressure drop (feed at 222 kPa) and a sweep gas (both sides at 138 kPa) configuration for membrane C2.
Membrane C2 separates the butane isomers with higher selectivities than membrane C1. As shown in Fig. 4, the two mixture selectivities exhibit similar temperature dependence, but the pressure drop selectivities are almost four times higher than those measured with a sweep gas. The highest selectivity is greater than 140. For both methods, the selectivity decreases to around 1.5 near 473 K. 3.3. Membrane T1 Membrane T1, which was prepared by a significantly different method without NaOH and on porous
Fig. 4. Mixture selectivities as measured using both a 138 kPa pressure drop (feed at 222 kPa) and sweep gas (both sides at 138 kPa) configuration for membrane C2.
stainless steel, exhibited qualitatively different permeation and selectivity behavior. Its N2 permeance (Table 1) is only 5% of that for membrane C1 and its N2 /SF6 ideal selectivity is much smaller. As shown in Fig. 5a, the single-gas and mixture permeances are almost identical for each isomer when measured by the pressure drop method. Thus, the ideal and separation selectivity trends do not vary significantly; n-butane does not decrease i-butane permeation in membrane T1. This is in sharp contrast to membranes C1 and C2. That is, unlike the other membranes, separation in membrane T1 is not due to preferential adsorption. The temperature dependencies for the two types of membranes are also quite different. The n-butane permeance is lower at room temperature for membrane T1, and it increases less with temperature before going through a maximum. The i-butane permeance is even more different since it increases by less than a factor of 2 over the temperature range in Fig. 5a (for single gas or mixture), but it increases by almost three orders of magnitude over the same temperature range for membrane C1 (Fig. 1a). Moreover, the n-butane single-gas permeances are greater than the i-butane permeances over the entire temperature range, in sharp contrast to Fig. 1a where i-butane permeated faster above 340 K. The permeance of n-butane in the mixture remains much larger than the permeance of i-butane over the entire temperature range for membrane T1. As shown in Fig. 5b, the same behavior was seen when a sweep gas was used, and the permeances in Fig. 5b are similar in magnitude to those in Fig. 5a. Preferential adsorption is not causing separations, and indeed, i-butane permeates faster in the mixture than as a single gas from 325 to 425 K. For n-butane, the sweep gas permeances are higher at low temperature and lower at high temperature than the pressure drop permeances. The general trends are the same for both methods, but the maximum is less pronounced with the sweep gas. For i-butane, the minimum in single-gas permeance is more distinct than for the pressure drop method, and no minimum exists for the mixture measurements. The differences between the single-gas and the mixture permeances were much larger when the permeate side of the membrane was under vacuum, as shown in Fig. 5c. The n-butane single-gas permeance is higher by a factor of 4 than the mixture permeance over the entire temperature range. The permeances
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
Fig. 5. Single-gas and mixture permeances for a n-butane/i-butane 50/50 mixture for membrane T1 using three experimental configurations: (a) a 138 kPa pressure drop (feed at 222 kPa), (b) a sweep gas (both sides at 138 kPa), and (c) an evacuated permeate (feed at 138 kPa).
43
for single-gas n-butane and mixture i-butane are 2–10 times larger than the corresponding permeances for the pressure drop and sweep gas methods. Although the difference is smaller at low temperature, the single-gas i-butane permeance is three times higher for the vacuum method than the other two techniques. The mixture-gas n-butane permeances are similar to those measured by the other two techniques. Also, in contrast to what is seen for n-butane, the i-butane permeance is significantly higher in the mixture than as a single gas. However, these measurements are less accurate than those made by the other experimental configurations. Since the permeate flow rate and composition could not be measured directly, they had to be calculated from mass balances. Also, the lower feed flow rate used may have affected the mass transfer on the feed side of the membrane. Single-gas i-butane permeances in both the pressure drop and sweep gas experiments exhibit minima around 340 K. When the permeate is evacuated, i-butane permeance remains low until about 400 K, after which it increases. Note also that the single-gas i-butane permeances are much lower than the single-gas n-butane permeances over the entire temperature range. As shown in Fig. 6a, the separation selectivities were not as high for membrane T1 as they were for membranes C1 and C2. Moreover, the effect of the measurement method is quite different, and membrane T1 still separates the isomers at 490 K. Whereas for the other membranes, the pressure drop method yielded the highest selectivities, the sweep gas method has much higher selectivity at low temperature for T1, and at higher temperature, the pressure drop and sweep are similar. The sweep gas method has a broad selectivity maximum but the pressure drop method has a sharper maximum. The vacuum selectivity is the lowest of the three, and remains relatively constant over the middle of the temperature range. The ideal selectivities are similar to the mixture selectivities for pressure drop and sweep measurements. However, as shown in Fig. 6b, the vacuum measurement yields the highest ideal selectivities, with the maximum being greater than 120. At high temperature, the ideal selectivities are all similar. Fig. 7 shows the n-butane single-gas flux through membrane T1 as a function of the pressure drop at 305 and 373 K. The permeate pressure was held constant at atmospheric pressure (84 kPa in Boulder, CO), and the
44
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
Fig. 7. Single-gas permeance of n-butane as a function pressure drop (permeate at 84 kPa) at 303 and 373 K for membrane T1. Solid lines indicate Maxwell–Stefan surface diffusion model.
q dp dq p dq dx dp KC KI +qIsat (4) = −ρε–D qCsat 1 + KC p 1 + KI p dx
N = −ρε–D
Fig. 6. (a) Separation selectivities for a 50/50 n-butane/i-butane mixture and (b) ideal n-butane/i-butane selectivities for membrane T1 using a 138 kPa pressure drop (feed at 222 kPa), sweep gas (both sides at 138 kPa), and evacuated permeate (feed at 138 kPa).
feed pressure was increased. The increase in flux with pressure is faster than first order for both temperatures over the pressure range studied. Also shown on the graph are predicted fluxes using the Maxwell–Stefan transport model based on Eqs. (1) and (2). A dual-site adsorption isotherm for n-butane (Eq. (3)) obtained from Zhu et al. [38] was used along with (Eq. (2)) to calculate the thermodynamic factor: q = qCsat
KC p KI p + qIsat 1 + KC p 1 + KI p
(3)
After combining Eqs. (1)–(3), the chain rule for multivariable functions was employed to transform the two derivatives into a single derivative of pressure with respect to position in the membrane (Eq. (4)):
Eq. (4) was then integrated over the change in partial pressure from the feed to the permeate side, and the thickness of the membrane. The proposed adsorption sites consist of the zig-zag and straight channels of the zeolite where n-butane prefers to adsorb, and the pore intersections where n-butane adsorption only significantly contributes to loading below 373 K. The isotherms were measured on silicalite-1 powder, but the aluminum content in membrane T1 is low (Si/Al=600). Adsorption isotherms measured by Dunne et al. [39,40] show that ZSM-5 adsorbs hydrocarbons slightly more strongly than silicalite, but that the general shape of the isotherms is the same on both materials. Therefore, the adsorption differences are probably small. The model was fitted by lumping the saturation coverage and diffusivity terms together to compensate for any difference in saturation. Any acceptable values would still be within 10% of those reported for silicalite. A 3 kPa pressure drop was imposed across a blank support to determine the importance of the mesoporous support resistance. The measured flux was used to perform a calculation using Darcy’s Law on the expected pressure gradient across the support in the pressure drop and vacuum experiments. For typical fluxes (∼1×10−4 mol/m2 /s),
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52 Table 2 Helium back-diffusion from permeate to retentate Helium back-flux (mmol/m2 /s)
Membrane
Temperature (K)
C1
301 479
2.8 27
C2
333 473
∼0 8.5
T1
300 489
∼0 0.1
the expected gradient was found to be less than 1 Pa, and therefore, ignored. The model fluxes are of a different functional form than the measured fluxes; the model predicts an increase that is slower than first order. Therefore, surface diffusion through zeolite pores alone does not describe the transport of butane isomers through membrane T1. 3.4. Stream compositions Table 2 lists the helium back-fluxes from the permeate side to the retentate side for the three membranes when using the sweep gas method. The amount of helium back-diffusion correlates with a membrane’s ability to separate the isomers. For the membranes with low penetration of the zeolite crystals into the support pores (C1, C2), the membrane with the lower maximum n-butane/i-butane selectivity (C1) has the higher helium back-flux to the retentate at high temperature. The membrane with more penetration of zeolite into the support (T1) has the lowest helium back-flux at high temperature. This membrane separates the isomers better at higher temperatures, but is also has lower fluxes for all molecules.
4. Discussion 4.1. Driving force calculations As stated previously, a log-mean pressure drop was used to calculate the partial pressure driving force for transport across the membrane because the feed composition changed as it flowed axially through the support. Although the difference between the log-mean and the arithmetic-mean partial pressure difference is small for the pressure drop method (2–6%), it can
45
be much larger for the sweep gas method. The lower pressures and the back-diffusion of helium from the permeate side can result in a significant change in the retentate partial pressures, yielding a difference in the calculated driving force of as much as 48% for a high flux membrane at high temperature. Since the permeate was considered to be well-mixed for the pressure drop and sweep gas methods, the log-mean pressure difference could be used simply as calculated. For the sweep gas method, the single pass countercurrent flow to cross-flow correction factor had to be considered [41]. However, butane partial pressures are low enough in the permeate stream to make the correction factor negligible. For typical experimental conditions, the correction factor was greater than 0.98, and therefore, ignored. 4.2. Types of membrane pores The membranes synthesized by the modified procedure of Coronas et al. [8,16] (C1, C2) display similar permeation behavior to those made by Coronas et al. when a pressure drop configuration was used. For single gases, i-butane permeates faster than n-butane over most of the temperature range used, while for mixtures n-butane permeates faster, at least up to 473 K. Membrane T1 shows a different behavior. The single- and mixture-gas permeances are almost identical for each gas for the pressure drop and sweep gas measurement methods, and the temperature dependencies are weaker than for membranes C1 and C2. Whereas the membranes made by the Coronas et al. procedure are not selective by 460 K, membrane T1 separates the isomers even at 490 K, where its selectivity is nine. This dramatic difference in permeation behavior results from a fundamental difference in the two types of membranes, and the most likely reason is a difference in pore structure. Gump et al. [9] studied the separation of hexane isomers using membranes prepared by the Coronas et al. procedure and proposed that these membranes separate because the linear isomer preferentially adsorbs into and blocks these nanosized, non-zeolite pores. Adsorbed n-hexane packed the non-zeolite pores, preventing the permeation of 2,2-dimethylbutane and resulting in permselectivities as high as 650. Similar pore blocking occurs with the butane isomers, although the effect is weaker due to the weaker
46
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
adsorption and smaller molecular size of the butanes. These membranes exhibit low ideal selectivities but high separation selectivities because separation is due to an interaction between the two isomers. The extent to which the interaction hinders the branched isomer controls the separation selectivity: smaller non-zeolite pores remain selective to higher temperatures at a given pressure. Such pore packing behavior has been simulated for nanopores. Yoshioka et al. [42] performed molecular dynamics simulations on the permeation of CO2 -like Leonard-Jones particles through an amorphous silica membrane. For 0.8 nm pores, the packing density of the molecules in the pores was low when the temperature was high. When the temperature was low, and especially below the critical temperature of the model gas, the pores became saturated with the diffusing molecules. A similar transport process could occur with the butane isomers. Such pore packing could be selective for the linear isomers due to the higher packing efficiency of the straight chains, as suggested by the molecular dynamics simulations of Vlugt et al. [43] for packing in zeolite pores. Entropic effects result in the ‘squeezing out’ of branched isomers at higher loadings. The pore structure of membrane T1 differs from that of the membranes C1 and C2. The permeances of both isomers are much weaker functions of temperature. Also, the single gas and mixture permeances of each isomer are similar, and they are similar functions of temperature for both the sweep gas and pressure drop methods. The membrane remains selective at high temperature, where adsorption is expected to play a smaller role. The isomers do not interact significantly, in contrast to membranes C1 and C2. The separation must therefore result from shape selectivity: the isomers diffuse at different rates and adsorb to different coverages. As measured by a variety of techniques, the diffusivity of n-butane through MFI pores is between 3 and 20 times larger than the diffusivity of i-butane [44–47]. The pure component adsorption coverages for the butane isomers on silicalite-1 [38] differ by less than a factor of three in the ranges of the pressure gradient experiments, and at low temperature both isomer loadings are nearly eight molecules per unit cell. Configurational-bias Monte Carlo simulations performed by Vlugt et al. [43] on silicalite-1 predict that the coverage of i-butane in a 50/50 mixture of the isomers at the conditions of the pressure gra-
dient experiments should drop to about one molecule per unit cell, whereas n-butane coverage in the mixture should remain near eight molecules per unit cell. However, this is not consistent with the behavior seen for the permeation of the two isomers through T1, or for the membrane reported by van de Graaf et al [20]. For membrane T1, the presence of n-butane has no effect or slightly increases the permeance of i-butane. Van de Graaf et al. reported a slight decrease in the permeance of i-butane in the mixture, but not nearly as large as predicted by the Monte Carlo simulations. Therefore, adsorption in the zeolite pores may not be the sole controlling mechanism in the separation. Other resistances, such as diffusion across intercrystalline boundaries, may also be important. The different pore structures of the two membrane types also explain the N2 /SF6 ideal selectivity behavior. The use of either a pressure gradient or sweep gas affects the permeation of SF6 through the membranes C1 and C2 just as it affects the permeation of the butanes. Lin et al. [33] measured the single-gas permeances of both N2 and SF6 through membranes like C1 and C2. The SF6 permeances were significantly higher when measured with a sweep gas, resulting in much smaller N2 /SF6 ideal selectivities. They concluded that SF6 adsorbs strongly in the membrane pores, resulting in a smaller concentration gradient and a lower permeance. But when a sweep gas is used, the SF6 adsorption coverage is lower on the permeate side, so that the concentration gradient is larger and thus the permeance is higher. The high N2 /SF6 ideal selectivities seen for membranes C1 and C2 most likely result from saturation of the non-zeolite pores with SF6 . Lai and Gavalas [48] reported N2 /SF6 ideal selectivities measured by a vacuum as high as 610 at 383 K for ZSM-5 membranes grown using a surface seeding technique. Although they performed no mixture separations, the membranes exhibited high ideal selectivities for the butane isomers, indicating that the membranes separated by shape selectivity. Sulfur hexafluoride permeates membrane T1 through the zeolite pores. At the temperatures studied, SF6 coverage is a strong function of pressure in these pores [39,40], resulting in a large enough gradient to drive permeation at a faster rate than through non-zeolite pores under similar conditions. The low N2 /SF6 ratio seen for T1 (19) is similar to the low values reported by other researchers who studied molecular sieving mem-
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
branes [36,37]. Because of these adsorption effects, the coverage gradient for SF6 is lower in the larger (non-zeolite) pores, resulting in higher N2 /SF6 ratios. Depending on the application, either pore type may be desirable. The nanosized non-zeolite pores achieve higher maximum selectivities since the presence of n-butane actively inhibits the transport of i-butane. Selectivities are lower in membrane T1 because the isomers interact less, but the zeolite pores maintain their selectivities to higher temperatures. The operating pressure also affects the two pore types differently. For separation of n-hexane and 2,2-dimethylbutane, the non-zeolite pores become more selective at higher pressures for the separation of hexane isomers [9], whereas the zeolite pores lose selectivity with higher pressure [7]. This, too, is due to the separation mechanism. At higher pressures, the linear isomer more effectively packs the non-zeolite membrane pores, resulting in a more effective ‘squeezing out’ of the branched isomer. For the zeolite pores, the higher pressures results in greater interaction between the two isomers. The diffusion of the linear isomer is slowed by the added resistance of the branched isomer in the pores, reducing the selectivity of the membrane. The pore structure of a membrane must therefore be tailored to the operating conditions. 4.3. Experimental configuration for C1 and C2 4.3.1. Pressure drop Changes in adsorption coverage explain the behaviors seen for membranes C1 and C2 for the different experimental configurations. Both isomers are able to enter the larger non-zeolite pores as pure components. For the pressure drop experiments, adsorption of the two species in the pores control the single-gas permeation. The partial pressure on both sides of the membrane is high (222 kPa on the feed, 84 kPa on the permeate). At low temperature both sides of the membrane are therefore nearly saturated for n-butane, but there is a significant coverage gradient for i-butane. The coverages for n-butane are higher, but the gradient is less. Therefore, even though n-butane has the larger diffusivity, i-butane permeates faster because of the larger concentration gradient and the ideal selectivities are less than one above 340 K. Van den Broeke [49] showed that surface coverage gradients could explain the higher single-gas permeance of i-butane through
47
zeolite pores. A similar process would explain the results for non-zeolite pores. For mixture separations using the pressure gradient method at temperatures below about 400 K, n-butane packs sufficiently in the pores to dramatically inhibit i-butane transport, but as the temperature increases, n-butane coverage decreases on both sides of the membrane. As the coverage decreases, i-butane can more readily permeate. The permeance of i-butane therefore remains low and nearly constant for membranes C1 and C2 until about 400 K, after which it increases rapidly. This is near the critical point of n-butane (425 K), where according to the molecular dynamics simulations of Yoshioka et al. [42], the CO2 -like particles in their molecular simulations packed tightly into the membrane pores. Single-gas and mixture permeations of n-butane are similar through such pores, since coverage is a weak function of pressure on both sides of the membrane. Because both sides are almost saturated, the driving force for diffusion is low because the adsorbed-phase concentration gradient is low. 4.3.2. Sweep gas When the single gas permeances are measured with a sweep gas, the partial pressures of the butane isomers on the permeate side of the membrane become a function of the permeation rate and the sweep gas flow rate. For the conditions used, the butane isomer partial pressures on the permeate side were typically less than 30 kPa, whereas the feed was maintained at 138 kPa. When pressures are less than 30 kPa, n-butane coverage depends more strongly on partial pressure, resulting in a larger concentration gradient across the membrane. The n-butane therefore permeates faster than the i-butane over the entire temperature range. The larger concentration gradient results in a higher permeance for n-butane than when a pressure drop is used (until 470 K), even through the partial pressure gradient is similar (less than 138 kPa for sweep gas, 138 kPa for pressure drop). The reverse is true for i-butane permeation. Because of the lower feed pressures when a sweep gas is used (138 kPa versus 222 kPa for pressure drop), the i-butane coverage is lower, resulting in a lower driving force for transport. Also, since i-butane does not pack the pores as well as n-butane, the back-diffusion of the He sweep gas may hinder the permeance of i-butane more than n-butane. Van de Graaf et al. [19] showed that the back-flux of
48
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
a helium sweep gas reduces the permeance of ethane through zeolite membranes. Shorter alkanes adsorb weaker than longer ones, so the hindering effect is expected to be smaller for n-butane than for ethane, but may still be important. The mixture separations are more similar for the pressure drop and sweep gas experiments. The permeance of n-butane as measured with pressure drop increases exponentially with temperature for both membranes, similar to the single-gas permeance of n-butane using a pressure drop. However, i-butane permeance exhibits two behaviors. When a pressure drop configuration was used, a minimum occurs in i-butane permeance at about 330 K for C1. This minimum may occur for C2, but no low temperature data were taken. When a sweep gas was used, i-butane permeance increased over the entire temperature range. The difference in i-butane permeance behavior is due to the degree of saturation of the membrane by n-butane. At the higher pressures used in the pressure drop experiments, the membrane pores contain more n-butane. Therefore a smaller fraction of the pores are available for i-butane transport. At the same time, the increasing temperature decreases i-butane coverage more than n-butane, causing the minimum in permeance. 4.4. Experimental configuration for T1 4.4.1. Pressure drop The permeation behavior of membrane T1 exhibits different dependencies on the experimental configuration since the transport occurs mainly through pores that separate by shape selectivity and diffusion. As seen in Fig. 5a, the pressure drop method shows little difference between single-gas and mixture permeances. The slightly lower permeances for n-butane in the mixture are probably due to the hindering of n-butane transport by the slower i-butane. At 300 K, the difference in feed partial pressure between the single-gas and mixture experiments does not affect the surface coverage; in both cases the n-butane loading on the feed side is about 1.4 mol/kg according to isotherm measurements by Zhu et al. [38]. As the temperature increases, however, the difference in surface coverages at the two pressures becomes more significant. At 473 K, there is about a 20% difference in n-butane loadings on either side of the membrane (0.51 versus 0.64 mol/kg [38]). The i-butane permeance exhibits
only a slight difference between the single gas and mixture at low temperature. Again, because the partial pressure of i-butane is higher in the pure component feed, the decrease in coverage due to the increasing temperature has a smaller effect on transport. Since the second adsorption site in the dual Langmuir model fit of the i-butane isotherm contributes significantly at low temperatures, the higher partial pressures in the single-gas experiments increase the coverages by about 10%. At about 373 K the difference in coverages at the two feed pressures is only about 1.5%. 4.4.2. Sweep gas Similarly, the sweep gas experiment (Fig. 5b) shows little difference between single-gas and mixture permeation of n-butane for membrane T1. The single-gas permeance of n-butane is slightly but consistently higher than the mixture permeance due to the added diffusive resistance of the i-butane in the mixture and the back-diffusion of helium, as reported by van de Graaf et al. [19] Compared to the pressure drop method, the sweep gas permeances are higher at low temperature, but lower at high temperature. On the feed side of the membrane, the different pressures for the two measurement methods result in small differences in coverage at low temperature. By 438 and 473 K, however, the coverages vary by 10 and 16%, respectively. On the permeate side, the partial pressures differ much more for the two experimental methods (84 kPa versus about 0.8 kPa), and the coverages are therefore significantly different. At low temperature, the coverage gradient is larger for the sweep gas experiment. For both experimental methods, the feed side is near saturation at 1.4 mol/kg, but the sweep gas permeate coverage is only 1.1 mol/kg, as compared to nearly 1.4 mol/kg for the pressure drop method. As the temperature increases, the difference in coverage on the permeate side for the two configurations increases. Since the permeate pressure for the sweep gas method is consistently in the Henry’s Law portion of the isotherm, the permeate coverage decreases more than in the pressure drop method, which not in the Henry’s Law portion of the isotherm. By 473 K, the lower pressures of the sweep gas method decrease surface coverages and coverage gradients, and the driving force for permeation becomes smaller. The higher pressures of the pressure
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
drop result in higher coverages and a larger gradient, and therefore the permeance is higher. The minimum in single-gas permeance exhibited by i-butane is indicative of a transition from one adsorption regime to another. As the temperature increases from 299 to 363 K, the coverage of i-butane decreases by about a factor of two, dominating the increase in diffusion rate and causing the permeance to decrease. But between 373 and 473 K when the coverage in the channels is negligible, the coverage decreases by only about 25%. The increase in diffusion rate then dominates the decrease in coverage in the intersections, and the permeance increases. In the mixture, the presence of n-butane could aid in the transport of i-butane at low temperature by preventing back-diffusion of helium from the sweep gas. 4.4.3. Vacuum Evacuation of the permeate side has a similar effect to the use of a sweep gas. The pressure of the permeate line was typically below 0.5 kPa, but was a function of the permeation rate, just as for the sweep gas. The surface coverage gradients for the vacuum configuration are therefore larger than those for the sweep gas configuration. For n-butane, the feed side coverage ranged from about 1.4 mol/kg at 303 K to 0.56 mol/kg at 473 K, while the coverages on the permeate side ranged from 1.1 to 0.006 mol/kg. The lower permeate pressures increase the concentration gradient, and the absence of helium reduces the resistance to diffusion for the permeating species. The permeances are therefore as much as seven times higher for the vacuum measurements than when measured with the sweep gas. For i-butane the coverages range from 1.2 to 0.38 kg/mol for the feed and from 0.5 to 0.004 kg/mol for the permeate, and the same arguments hold true. In both cases, the gradients are larger than for the pressure drop configuration, and there is less resistance to flow than for the sweep gas method. As a result, the single-gas permeances for both butane isomers are highest using the vacuum configuration. The same driving force arguments explain why the n-butane mixture permeances are highest in the vacuum configuration. 4.4.4. Selectivities The trends for the different separation and ideal selectivities show that the experimental configuration
49
must be taken into account when evaluating and comparing membranes. The separation selectivities for membrane T1 increase in the order vacuum
50
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
layer. Therefore, the surface diffusion process does not model the transport through our membranes. 4.5. Helium back-diffusion When a sweep gas was used, the degree of helium back-diffusion provides an indication of membrane pore structure. The back-diffusion is measured by the helium flux from the permeate side of the membrane to the retentate (Table 2), as calculated from the difference in the total moles in a retentate sample and the moles of n- and i-butane in the sample as measured by the GC. At low temperature, the zeolite and small non-zeolite pores in both types of membranes should be saturated with n-butane, and helium should not back-diffuse through them. Helium in the retentate must back-diffuse through the larger non-zeolite pores. As the temperature increases, the butanes desorb from the non-zeolite pores more readily than from the zeolite pores. The irregular structure of the non-zeolite pores and their larger size as compared to the zeolite pores provides helium with more room to pass a butane molecule in the pore. The helium back-diffusion should therefore depend strongly on temperature. The butane packs into zeolite pores more readily and tightly, resulting in little room for a helium molecule to get by. The coverage of butane in the pores must drop significantly before an appreciable amount of helium can back-diffuse. For membrane C1, the back-flux is appreciable at room temperature, and increases by almost an order of magnitude by 479 K. These values are significantly higher than the helium back-fluxes for membrane C2, for which helium back-flux is below detectable levels at room temperature. This is added evidence that the non-zeolite pores of membrane C1 are larger than those of membrane C2. The membrane with the largest fraction of transport through zeolite pores, T1, allows the least amount of helium back-diffusion. The back-flux is below detectable levels at 300 K and is almost two orders of magnitude lower than the back-flux of helium for membrane C2 at high temperature, and an order of magnitude lower than for membrane C2 when normalized by the permeances of the membranes. Not only is the back-flux lower, but it is a much weaker function of temperature. The extent of helium back-diffusion therefore provides information concerning the fraction of non-zeolite pores in the membrane.
5. Conclusions ZSM-5 zeolite membranes with non-zeolite pores can separate n-butane/i-butane mixtures with high selectivities by preferential adsorption of n-butane. Separation selectivities were as high as 140, but the ideal selectivities were significantly lower. Since adsorption coverages are low at high temperature, these membranes lost most of their selectivity above about 460 K. For ZSM-5 membranes with mostly zeolite pores, single-gas and mixture permeances were similar. Differences in adsorption coverages and diffusion rates control the separation, so separation selectivities were maintained at high temperature. The method of measurement has a large effect on the permeances and selectivities. At high pressures, i-butane permeates through non-zeolite pores faster than n-butane as a single gas because i-butane has a larger concentration gradient across the membrane. For mixtures, the preferential adsorption of n-butane is more effective at higher pressures. Permeation of the butane isomers through zeolite pores depends less on the experimental configuration. Single- and mixture-gas permeances for the three configurations differ slightly in magnitude, but display the same general trends.
Acknowledgements Acknowledgment is made of the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We also gratefully acknowledge support by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. We thank Dr. Vu A. Tuan for synthesizing membrane T1. References [1] D.M. Ruthven, S. Farooq, K.S. Knaebel, Pressure Swing Adsorption, VCH Publishers, New York, 1994. [2] R.T. Yang, Gas Separation by Adsorption Processes, Butterworths, Stoneham, MA, 1987. [3] E.M. Flanigen, J.M. Bennett, R.W. Grose, J.P. Cohen, R.L. Patton, R.M. Kirchner, Silicalite, a new hydrophobic crystalline silica molecular sieve, Nature 271 (1978) 512. [4] A. Giroir-Fendler, J. Peureux, H. Mozzanega, J.A. Dalmon, Characterization of a zeolite membrane for catalytic
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
membrane reactor application, Stud. Surf. Sci. Catal. 101 (1996) 127. Z.A.E.P. Vroon, K. Keizer, M.J. Gilde, H. Verweij, A.J. Burggraaf, Transport properties of alkanes through ceramic thin zeolite MFI membranes, J. Membr. Sci. 113 (1996) 293. K. Keizer, A.J. Burggraaf, Z.A.E.P. Vroon, H. Verweij, Two component permeation through thin zeolite MFI membranes, J. Membr. Sci. 147 (1998) 159. C.L. Flanders, V.A. Tuan, R.D. Noble, J.L. Falconer, Organic isomer separations by ZSM-5 membranes, J. Membr. Sci. (2000), submitted for publication. J. Coronas, R.D. Noble, J.L. Falconer, Separations of C4 and C6 isomers in ZSM-5 tubular membranes, Ind. Eng. Chem. Res. 37 (1998) 166. C.J. Gump, R.D. Noble, J.L. Falconer, Separation of hexane isomers through non-zeolite pores in ZSM-5 zeolite membranes, Ind. Eng. Chem. Res. 38 (1999) 2775. G. Cao, Y. Lu, L. Delattre, C. Brinker, G. Lopez, Amorphous silica molecular sieving membranes by sol–gel processing, Adv. Mater. 8 (1996) 588. N. Nishiyama, T. Matsufuji, K. Ueyama, M. Matsukata, FER membrane synthesized by a vapor-phase transport method: its structure and separation characteristics, Microporous Mater. 12 (1997) 293. A.J. Burggraaf, Z.A.E.P. Vroon, K. Keizer, H. Verweij, Permeation of single gases in thin zeolite MFI membranes, J. Membr. Sci. 144 (1998) 77. Y. Yan, M.E. Davis, G.R. Gavalas, Preparation of zeolite ZSM-5 membranes by in-situ crystallization on porous ␣-Al2 O3 , Ind. Eng. Chem. Res. 34 (1995) 1652. K. Kusakabe, A. Murata, T. Kuroda, S. Morooka, Preparation of MFI-type zeolite membranes and their use in separating n-butane and i-butane, J. Chem. Eng. Jpn. 30 (1997) 72. T. Masuda, A. Sato, H. Hara, M. Kouno, K. Hashimoto, Preparation of a dense ZSM-5 zeolite film on the outer surface of an alumina ceramic filter, Appl. Catal. A 111 (1994) 143. J. Coronas, J.L. Falconer, R.D. Noble, Characterization and permeation properties of ZSM-5 tubular membranes, AIChE J. 43 (1997) 1797. K. Kusakabe, T. Kuroda, A. Murata, S. Morooka, Formation of a Y-type zeolite membrane on a porous ␣-alumina tube for gas separation, Ind. Eng. Chem. Res. 36 (1997) 649. M. Matsukata, E. Kikuchi, Zeolitic membranes: synthesis, properties and prospects, Bull. Chem. Soc. Jpn. 70 (1997) 2341. J.M. van de Graaf, F. Kapteijn, J.A. Moulijn, Methodological and operational aspects of permeation measurements on silicalite-1 membranes, J. Membr. Sci. 144 (1998) 87. J. van de Graaf, E. van der Bijl, A. Stol, F. Kapteijn, J.A. Moulijn, Effect of operating conditions and membrane quality on the separation performance of composite silicalite-1 membranes, Ind. Eng. Chem. Res. 37 (1998) 4071. M.C. Lovallo, A. Gouzinis, M. Tsapatsis, Synthesis and characterization of oriented MFI membranes and films prepared by secondary growth, AIChE J. 44 (1998) 1903. L.J.P. van den Broeke, F. Kapteijn, J.A. Moulijn, Transport and separation properties of a silicalite-1 membrane. II. Variable separation factor, Chem. Eng. Sci. 54 (1999) 259.
51
[23] Z.A.E.P. Vroon, Synthesis and Transport studies of thin Ceramic Supported Zeolite (MFI) Membranes, Ph.D. Thesis, University of Twente, Twente, 1995. [24] J.C. Poshusta, R.D. Noble, J.L. Falconer, Temperature and pressure effects on CO2 and CH4 permeation through MFI zeolite membranes, J. Membr. Sci. 160 (1999) 115. [25] E.R. Geus, H.V. Bekkum, W.J.W. Bakker, J.A. Moulijn, High-temperature stainless steel supported zeolite (MFI) membranes: preparation, module construction, and permeation experiments, Microporous Mater. 1 (1993) 131. [26] R. Krishna, A unified approach to the modeling of intraparticle diffusion in adsorption processes, Gas Sep. Purif. 7 (1993) 91. [27] F. Kapteijn, W.J.W. Bakker, G. Zheng, J.A. Moulijn, Temperature- and occupancy-dependent diffusion of n-butane through a silicalite-1 membrane, Microporous Mater. 3 (1994) 227. [28] A.J. Burggraaf, Single gas permeation of thin zeolite (MFI) membranes: theory and analysis of experimental observations, J. Membr. Sci. 155 (1999) 45. [29] R. Krishna, L.J.P. van den Broeke, The Maxwell–Stefan description of mass transport across zeolite membranes, Chem. Eng. J. 57 (1995) 155. [30] W.J.W. Bakker, F. Kapteijn, J. Poppe, J.A. Moulijn, Permeation characteristics of a metal-supported silicalite-1 zeolite membrane, J. Membr. Sci. 117 (1996) 57. [31] V.A. Tuan, J.L. Falconer, R.D. Noble, Alkali-free ZSM-5 membranes: preparation conditions and separation performance, Ind. Eng. Chem. Res. 38 (1999) 3635. [32] R.W. Grose, E.M. Flanigen, Crystalline Silica, US Patent 4,061,724 (1977). [33] X. Lin, J.L. Falconer, R.D. Noble, Parallel pathways for transport in ZSM-5 zeolite membranes, Chem. Mater. 10 (1998) 3716. [34] Z.A.E.P. Vroon, K. Keizer, A.J. Burggraaf, H. Verweij, Preparation and characterization of thin zeolite MFI membranes on porous supports, J. Membr. Sci. 144 (1998) 65. [35] H.H. Funke, M.G. Kovalchick, J.L. Falconer, R.D. Noble, Separation of hydrocarbon isomer vapors with silicalite zeolite membranes, Ind. Eng. Chem. Res. 35 (1996) 1575. [36] W.J.W. Bakker, L.J.P.v.d. Broeke, F. Kapteijn, J.A. Moulijn, Temperature dependence of one-component permeation through a silicalite-1 membrane, AIChE J. 43 (1997) 2203. [37] G. Xomeritakis, A. Gouzinis, S. Nair, T. Okubo, M.Y. He, R.M. Overney, M. Tsapatsis, Growth, microstructure, and permeation properties of supported zeolite (MFI) films and membranes prepared by secondary growth, Chem. Eng. Sci. 54 (1999) 3521. [38] W. Zhu, J.M. van de Graaf, L.J.P. van den Broeke, F. Kapteijn, J.A. Moulijn, TEOM: a unique technique for measuring adsorption properties — light alkanes in silicalite-1, Ind. Eng. Chem. Res. 37 (1998) 1934. [39] J.A. Dunne, R. Mariwala, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric heats of adsorption and adsorption isotherms. 1. O2 , N2 , Ar, CO2 , CH4 , C2 H6 , and SF6 on silicalite, Langmuir 12 (1996) 5888.
52
C.J. Gump et al. / Journal of Membrane Science 173 (2000) 35–52
[40] J.A. Dunne, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric heats of adsorption and adsorption isotherms. 2. O2 , N2 , Ar CO2 , CH4 , C2 H6 , and SF6 on NaX, H-ZSM-5, and Na-ZSM-5 zeolites, Langmuir 12 (1996) 5896. [41] F. Incropera, D. DeWitt, Fundamentals of Heat and Mass Transfer, 4th Edition, Wiley, New York, 1996. [42] T. Yoshioka, T. Tsuru, M. Aseada, Molecular dynamics simulations of gas permeation in microporous silica membranes, in: Proceedings of International Congress on Membranes, Toronto, Canada, 1999. [43] T.J.H. Vlugt, R. Krishna, B. Smit, Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite, J. Phys. Chem. B 103 (1999) 1102. [44] M.S. Sun, O. Talu, D.B. Shah, Diffusion measurements through embedded zeolite crystals, AIChE J. 42 (1996) 3001. [45] A. Paravar, D. Hayhurst, Direct measurement of diffusivity for butane across a single large silicalite crystal, in: Proceedings
[46]
[47]
[48] [49]
[50]
of the 6th International Conference on Zeolites, Guildford, 1984, p. 217. A. Chaing, A. Dixon, Y. Ma, The determination of zeolite crystal diffusivity by gas chromatography. II. Experimental, Chem. Eng. Sci. 39 (1984) 1461. J.R. Hufton, D.M. Ruthven, Diffusion of light alkanes in silicalite studied by the zero length column method, Ind. Eng. Chem. Res. 32 (1993) 2379. R. Lai, G.R. Gavalas, Surface seeding in ZSM-5 membrane preparation, Ind. Eng. Chem. Res. 37 (1998) 4275. L.J.P. van den Broeke, Contribution of adsorption equilibrium to permeation through MFI zeolite membranes, personal communication. L.J.P. van den Broeke, W.J.W. Bakker, F. Kapteijn, J.A. Moulijn, Transport and separation properties of a silicalite-1 membrane. I. Operating conditions, Chem. Eng. Sci. 54 (1999) 245.